1. Don't underestimate the importance of knowing the character spine.
Truly great characters have an innate consistency that can only come from knowing the character's inner workings. Spend the time developing the spine. Your characters will be better differentiated.

2. Try writing without character names.
This is a well-guarded professional secret used by hundreds of pros. When you write your characters, don't write who's saying what (you can add it later). This way, you are sure to make each character speak in different voices just to keep it straight in your own mind as you write. It forces you to make each character unique.

3. Avoid basing characters on friends and family.
Besides the obvious political implications (Thanksgiving is bad enough), when a writer does this, he or she is basically mimicking the outer traits of a person. You never truly know how your characters tick.

4. Make your hero and your villain the same personality type.
This way the hero and villain are fighting for the same basic things and have an innate understanding of each other -- only the hero is coming from a healthier place. A moment of growth comes for the hero when he realizes he could be like the bad guy if he didn't have inner strength. He literally is overcoming an evil element in himself when he defeats the villain.

5. Study the personality types generally used in your genre.
For instance, the "Boss" personality type in Character Writer is usually the hero in an action movie. This can clue you into which personality types work great with which genres.

6. Ignore Tip #5.
Sometimes putting a personality type that's not usually in that genre can create an incredibly unpredictable and wonderful story dynamic. In fact, personally, I'd advise using a type not usually found in your genre to shake things up.

7. Make your characters consistent.
All your characters should differentiate themselves from each other and do it very consistently throughout the entire story. Break it, and you'll lose your audience. Keep notes on your character as you write (easy to do with Character Writer's Writing Mode feature).

8. Study dialogue styles.
Each type has its own style of talking. Make sure you define that style before taking on writing dialogue. It will keep it true to the character.

9. Try to think of every character as starring in their own story.
You have a hero you're focusing all your attention on. But, to get truly interesting supporting characters think about what their story would be. Could you write a different story focusing on them?

10. Play with your character's expectations.
Different people expect different things to happen in certain situations. Some believe they'll fail at whatever they try and others assume victory. Define what those expectations are... then, make sure things don't work out that way. It makes for great scenes and great characters.